Nano-additives enabled advanced lubricants

ABSTRACT

The presently disclosed technology relates to a nano-additives to improve the performance of lubricants, oils, and greases. More specifically, the presently disclosed technology relates to applying capped metal oxide nanoparticles, such as capped zirconia nanoparticles, in the lubricants to produce a tribofilms on the lubricating surfaces to provide wear protection to the said surfaces. Also, the interaction of the capped zirconia nanoparticles with other commonly used additives in lubricants may further optimize the performance of the resulting tribofilms.

This application is a continuation of U.S. application Ser. No.15/569,271 filed on May 4, 2016 (now U.S. Pat. No. ______), which inturn is the U.S. national phase of International Application No.PCT/US2016/030678 filed May 4, 2016, which designated the U.S. andclaims benefit of U.S. Provisional Application Nos. 62/156,400,62/163,116, 62/163,126, filed May 4, 2015, May 18, 2015 and May 18,2015, respectively, the entire contents of each of which are herebyincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This application is partially supported by a US Dept. of EnergyCorporate Research and Development Agreement (CRADA) No. 1200801 and USDept. of Energy Small Business Innovation Research (SBIR) Phase I and IIGrants No. DE-SC0009222.

This presently disclosed technology pertains, among other things, to alubricant containing nano-additives for oils and greases. The presentdisclosure provides a zirconia nanoparticles dispersion in oils with orwithout other additives. The function of these nano-additives are toform a protective tribofilm on contacting surfaces. The tribofilm maysupplement the boundary and fluid film formed by the lubricant toprovide wear and/or friction reduction and thus enable the use oflubricants with lower viscosity.

Lubricating oils and greases are commonly used in a variety ofapplications, for example: crankcase lubricants for internal combustionengines, lubricating oils for geared transmissions in vehicles and windturbine drivetrains, and grease or oil lubricants for rolling elementbearings. The lubricant provides protection against, among other damageincluding corrosion, wear of the contacting surfaces through apressurized fluid film and/or the formation of a solid tribofilmgenerated during operation. While a fluid film is governed by theviscosity of the oil, the tribofilm formation is typically provided bychemical additives that react to form a solid film on the surface. Inefforts to improve efficiency of mechanical drives there is a trend toreduce the viscosity of the lubricating oils to lower the churning orviscous loses. To maintain the durability of components, the performancerequirements of the lubricant additives are more demanding, specificallyfor friction and wear.

The chemical additives traditionally used in lubricants to provideprotective tribofilms are referred to as Anti-Wear (AW) and ExtremePressure (EP) additives. Furthermore Friction Modifiers (FM) are used tomaintain a low shear surface at the contact. These additives come in avariety of forms but most are organometallic compounds containingphosphorus, sulfur, and zinc. These compounds chemically react with thecontacting surfaces to form an amorphous and/or crystalline solidtribofilm. While the mechanisms responsible for tribofilm formation fromorganometallics is still a topic of ongoing research, in practice it isgenerally observed that a certain level of shear, pressure, and/ortemperature is required to nucleate and grow a tribofilm withorganometallic compounds. Furthermore, in automotive applications, thephosphorus and sulfur content of these additives have been shown to havea detrimental impact on the exhaust after treatment catalysts; this hasled to tighter restrictions on allowable content of these compounds inthe lubricant.

The use of nanoparticles as an additive to lubricants to provide AW, EP,and FM performance qualities presents an innovative approach tosupplement or replace the use of organometallic compounds or otheradditive chemistries. The mechanisms governing the formation of atribofilm from nanoparticles are fundamentally different than those ofthe chemical additives, which presents potential advantages in certaincontact configurations. Therefore, inorganic nanoparticles, particlesless than 100 nm in diameter, have recently been a subject of interestas friction modifier or anti-wear agent for lubricants. There have beenmany studies on the subject (H. Spikes, Lubr. Sci. 20 (2008), pp.103-136; J. Tannous et al., Tribol. Lett. 41 (2011), pp 55-64; A.Hernandez Batterz et al., Wear 265 (2008), pp. 422-428; H. Kato and K.Komai, Wear 262 (2007), pp 36-41). These studies, however, all sufferfrom (1) the lack of control on the quality of nanoparticles, i.e. thesize and size distribution, and (2) lack of dispersion stability in theoils. The results, therefore, were not conclusive regarding the benefitthe nanoparticle additives provided. It is now understood that to enablethe advantages provided by nanoparticle additives and to avoid anydetrimental consequences, the nanoparticles have to meet certainconsiderations include: dispersion and suspension, stability at elevatedtemperature, compatibility and synergy with other lubricant additives,and interaction compatibility with contacting surfaces.

In the past few years, Pixelligent has developed a family of inorganicnanoparticles and nanocrystals which have small size (typically smallerthan 10 nm diameter), with a narrow size distribution, and mostimportantly, an engineered surface chemistry so that they can bedispersed into common base stocks without observable impact on theappearance, viscosity, and shelf-life of the oils. Nanoparticles will beunderstood to include nanocrystals. Because Pixelligent's nanoparticlesare much smaller than typical asperities of almost all practicalmanufactured surfaces in tribological applications, and also because ofthe quality and stability of the dispersion, true nano-scale control ofthe tribological behavior has been observed, and the benefits of thenanoparticle additives can be leveraged for reducing friction and wear.

This presently disclosed technology provides, among other things, that azirconia nanoparticle dispersion in oils with or without other additivesforms a protective tribofilm that is self-limiting and self-regeneratingin rolling, sliding, or rolling-sliding contact. This is achievedthrough well-dispersed, capped nanoparticles to maintain a stable,homogeneous distribution and avoiding agglomeration of particles. Thenano-scale size of the particle, 4-20 nm, is critical in enabling theadditive to enter the contact while avoiding any unintended detrimentaleffects. If the nanoparticles are not capped or dispersed attractiveforces bring the particles together causing agglomeration and leading tofall-out of suspension. The agglomerations lead to a non-uniform mixturein the oil and if the agglomeration is large and hard enough can lead toabrasion of the contacting surface resulting in increased wear.

In addition to having a well dispersed nanoparticle that enters thecontact, this presently disclosed technology provides a nanoparticlethat, once in contact, adheres strongly to the component surface andgrows a thick tribofilm (30 nm to 500 nm). The nucleation of thistribofilm occurs in sliding, rolling, or rolling-sliding contacts, andat temperature ranges of −50° C. to 160° C. and beyond, thus extendingthe conditions that traditional AW and EP additives form tribofilms.

The present disclosure provides nano-additives for lubricants, oils, andgreases. During operation, the said nano-additive may build protective,self-limiting, self-regenerating tribofilms in rolling, sliding, orrolling-sliding contacts. Such a tribofilm may reduce wear and/orfriction at the lubricating contacts. Such a tribofilm may supplementthe boundary, mixed, elasto-hydrodynamic (EHL) and/or hydrodynamic filmformed by the lubricant thus allowing lubricant viscosity reduction.

The presently disclosed lubricants, oils, and greases may include anymineral and synthetic oils including synthetic hydrocarbons, esters,polyglycols, silicones, and ionic liquids.

The present disclosure provides a zirconia nanoparticle dispersion, inpure oils or oils with other lubricant additives comprising anti-wear(AW) additives such as zinc dialkyldithiophosphates (ZDDP), or frictionmodifiers (FM), anti-oxidants, extreme pressure (EP) additives,anti-foams, detergents, dispersants, pour point depressants, or anyother commonly used lubricant additives.

The presently disclosed zirconia nanoparticles may be capped withsurface capping agents as previously described in any of U.S. Pat. Nos.8,883,903; 9,328,432; 9,202,688 and 8,920,675, the entire contents ofeach of which are incorporated herein by reference.

The presently disclosed zirconia nanoparticles may have size smallerthan 20 nm, or smaller than 15 nm, or smaller than 10 nm, or smallerthan 5 nm.

The presently disclosed zirconia nanoparticle dispersion may demonstratehigher clarity. Said dispersion with 10 wt % capped zirconiananoparticles, when measured in a cuvette with 10 mm optical path,demonstrates optical transmittance higher that 50%, or higher than 60%,or higher than 70%, or higher than 80%, or higher than 90%, or higherthan 95%, or higher than 99%.

The presently disclosed zirconia nanoparticle dispersion may demonstratehigh stability. Said dispersion with 10 wt % capped zirconiananoparticles, when measured in a cuvette with 10 mm optical path,demonstrates change in optical transmittance less than 10%, or less than5%, or less than 1%, after 1 month storage, or after 3 month storage, orafter 6 month storage, or after 1 year storage, or after 2 year storage,or after 3 year storage.

The presently disclosed zirconia nanoparticles may form a tribofilm ontribologically contacting surfaces in relative motion and undertribological stress. Said tribofilm may be highly dense andpolycrystalline. Said tribofilm may have thickness in the range of 30 nmto 500 nm. Said tribofilm may have a hardness less than or equal to 7.3GPa, and modulus less than or equal to about 160 GPa when measured withnano-indentation.

The small size and superb dispersibility of the nanoparticles enablethem to enter the space separating asperities on the surfaces in atribological contact. The mechanism of the tribofilm formation may bethat under tribological stress, the capping agents on the nanoparticlesurface are removed, the nanoparticles are bonded to the rubbingsurfaces to form nucleation sites, the nanoparticles coalesce onto thenucleation sites, and then undergo grain coarsening to form an integraltribofilm. The tribofilm growth is stress driven and higher stress leadsto faster nucleation and tribofilm growth process.

The presently disclosed tribofilm may demonstrate self-limitingthickness during its formation under a given tribological condition. Themaximum film thickness may be 30 nm-50 nm, or 50 nm-100 nm, or 100nm-200 nm, or 200 nm-300 nm, or 300 nm-400 nm, or 400 nm-500 nm, or 500nm or larger.

The presently disclosed tribofilm may have surface RMS roughness equalto or less than 2 nm, or 2 nm-5 nm, or 5 nm-10 nm, or 10 nm-50 nm, or 50nm-100 nm, or 100 nm-500 nm.

The presently disclosed tribofilm has carbon content of 10%-15%, or5%-10%, or less than 5%, as measured by EDX, EELS, or FTIR.

The presently disclosed tribofilm may have high adhesion to thesubstrates as measured the by tape test.

The presently disclosed tribofilm may not be removed by acid such as 10%hydrochloric acid solution, or base, such as 10% tetramethylammoniumhydroxide (TMAH) solution.

The presently disclosed tribofilm may form under pure sliding, purerolling, or mixed rolling-sliding conditions.

The presently disclosed tribofilm may form in the temperature range of−50 C to 160 C, or 0 C to 160 C, or 20 C to 130 C.

The presently disclosed tribofilm may form on a steel surface, or asilicon surface, an amorphous carbon surface or a ceramic such asyttria-stabilized zirconia surface.

The presently disclosed tribofilm may form on surfaces with RMS surfaceroughness larger than 5 nm.

The presently disclosed tribofilm may form with an oil with 10 wt %capped ZrO2 nanoparticles, or 1 wt % capped ZrO2 nanoparticles, or 0.1wt % capped ZrO2 nanoparticles, or 0.01 wt % capped ZrO2 nanoparticles.

The presently disclosed tribofilm may form under tribological contact 10nm or wider, or 1 um or wider, or 150 um or wider, or 1 mm or wider.

The presently disclosed tribofilm may be formed in the presence of ZrO2nanoparticles together with anti-wear (AW) additives such as zincdialkyldithiophosphates (ZDDP), or friction modifiers (FM),anti-oxidants, extreme pressure (EP) additives, anti-foams, detergent,dispersants, pour point depressants, or any other commonly usedlubricant additives.

The presently disclosed technology provides a method of forming a solidfilm on a lubricated surface that includes placing a lubricant in acontact region defined by two surfaces in proximity, sliding and/orrolling said surfaces so as to produce a pressure and/or shear stress onthe lubricated surface in the contact region, and thereby forming thesolid film in the contact region, wherein the solid film is adhered toat least one of the surfaces in the contact region, the lubricantcontaining at least partially capped, metal oxide nanocrystals.

Metal oxide nanocrystals of the presently disclosed technology includezinc oxide, hafnium oxide, zirconium oxide, hafnium-zirconium oxide,titanium-zirconium oxide and/or yttrium oxide.

Methods of the presently disclosed technology provide solid films thatpersists after formation and in the absence of said sliding and/orrolling forces.

Pressures useful in methods of the presently disclosed technology mayrange from 100 MPa to 5 GPa, 100 MPa to 200 MPa, 200 MPa to 400 MPa, 400MPa to 800 MPa, 800 MPa to 1.5 GPa, 1.5 GPa to 3 GPa, 3 GPa to 5 GPa or5 GPa to 10 GPa.

Shear stresses useful in methods of the presently disclosed technologymay range from 10 MPa to 0.5 GPa, 10 MPa to 100 MPa, 100 MPa to 200 MPa,200 MPa to 500 MPa, or 500 MPa to 1 GPa.

Methods of the presently disclosed technology provide or includelubricants having at least partially capped nanocrystals in an amount of0.01 to 2 percent by weight of the lubricant, 0.01 to 0.05 percent byweight of the lubricant, 0.05 to 0.1 percent by weight of the lubricant,0.1 to 0.2 percent by weight of the lubricant, 0.2 to 0.3 percent byweight of the lubricant, 0.3 to 0.4 percent by weight of the lubricant,0.4 to 0.5 percent by weight of the lubricant, 0.5 to 0.75 percent byweight of the lubricant, 0.75 to 1 percent by weight of the lubricant, 1to 1.5 percent by weight of the lubricant, 1.5 to 2 percent by weight ofthe lubricant, or 2 to 10 percent by weight of the lubricant.

Methods of the presently disclosed technology involve or includeformation of the solid film at a temperature in a contact region duringthe sliding and/or rolling in the range of −100° C. to 200° C., −100° C.to −50° C., −50° C. to −25° C., −25° C. to 0° C., 0° C. to 10° C., 10°C. to 20° C., 20° C. to 30° C., 30° C. to 40° C., 40° C. to 50° C., 50°C. to 60° C., 60° C. to 70° C., 70° C. to 80° C., 80° C. to 90° C., 90°C. to 100° C., 100° C. to 125° C., 125° C. to 150° C., 150° C. to 175°C., 175° C. to 200° C.

Lubricants of the presently disclosed technology may include a ZDDPadditive, optionally present in an amount of 0.01 to 2 percent by weightof the lubricant, 0.01 to 0.05 percent by weight of the lubricant, 0.05to 0.1 percent by weight of the lubricant, 0.1 to 0.2 percent by weightof the lubricant, 0.2 to 0.3 percent by weight of the lubricant, 0.3 to0.4 percent by weight of the lubricant, 0.4 to 0.5 percent by weight ofthe lubricant, 0.5 to 0.75 percent by weight of the lubricant, 0.75 to 1percent by weight of the lubricant, 1 to 1.5 percent by weight of thelubricant, 1.5 to 2 percent by weight of the lubricant, or 2 to 10percent by weight of the lubricant.

Methods of the presently disclosed technology include forming the solidfilm on at least one surface or two surfaces that contains a steelcomposition.

Methods of the presently disclosed technology are able to form films andfilms formed according to the presently disclosed technology have a filmhardness of 1 to 20 GPa, 100 MPa to 200 MPa, 200 MPa to 500 MPa, 500 MPato 750 MPa, 750 MPa to 1 GPa, 1 GPa to 2 GPa, 2 GPa to 3 GPa, 3 GPa to 5GPa, 5 GPa to 7 GPa, 7 GPa to 10 GPa, 10 GPa to 15 GPa, 15 GPa to 20GPa.

Methods of the presently disclosed technology are able to form films andfilms formed according to the presently disclosed technology haveYoung's modulus of 50 GPa to 300 GPa, 50 GPa to 75 GPa, 75 GPa to 100GPa, 100 GPa to 125 GPa, 125 GPa to 150 GPa, 150 GPa to 200 GPa, or 200GPa to 250 GPa.

Methods according to the presently disclosed technology may involve orinclude a sliding or rolling of the surfaces in the contact region toinduce a shear rate on the lubricant in the range of 0 to 10⁷ sec⁻¹, 0to 10² sec⁻¹, 10² to 10³ sec⁻¹, 10³ to 10⁴ sec⁻¹, 10⁴ to 10⁵ sec⁻¹, 10⁵to 10⁶ sec⁻¹, or 10⁶ to 10⁷ sec⁻¹, or a shear rate that induces atribological shear stress.

Methods of the presently disclosed technology further optionally includeor involve formation of an elasto-hydrodynamic lubricant (EHL) filmand/or a boundary lubricant film and/or hydrodynamic lubricant film inthe contact region.

Lubricants included in the methods of the presently disclosed technologyand films formed by the methods may be an oil or a grease, or asynthetic, mineral or a natural lubricant, or contain at least one of asynthetic hydrocarbon, an ester, a silicone, a polyglycol or an ionicliquid, or is an oil having a viscosity in the range of 2 to 1000 mPas(cP), 2 cP to 10 cP, 10 cP to 50 cP, 50 cP to 100 cP, 100 cP to 500 cP,or 500 cP to 1000 cP, at a temperature of 100° C.

Methods of the presently disclosed technology and films provided by thepresently disclosed technology may include lubricants containing atleast one of an anti-wear (AW) additive, a friction modifier such aszinc dialkyldithiophosphates (ZDDP), or friction modifiers (FM),anti-oxidants, extreme pressure (EP) additives, anti-oxidants,anti-foams, detergents, dispersants, pour point depressants, or anyother commonly used lubricant additives.

The presently disclosed technology provides a solid film on a lubricatedsurface containing a metal oxide crystallite, the crystallite having amean size of 5-20 nm, 5 to 100 nm, 5 nm to 10 nm, 10 nm to 20 nm, 20 nmto 30 nm, 30 nm to 40 nm, 40 nm to 50 nm, 50 nm to 60 nm, 60 nm to 70nm, 70 nm to 80 nm, 80 nm to 90 nm, or 90 nm to 100 nm, the film havingan atomic ratio of carbon to metal in the range of 0 to 0.05, or 0.05 to0.1, or 0.1 to 0.15, or 0.15 to 0.2, or 0.2 to 0.25, or 0.25 to 0.3, or0.3 to 0.35, or 0.35 to 0.4, or 0.1 to 0.4.

Solid films of the presently disclosed technology optionally have athickness of 20 to 500 nm, 20 nm to 50 nm, 50 nm to 100 nm, 100 nm to200 nm, 200 nm to 300 nm, 300 nm to 400 nm, or 400 nm to 500 nm.

Solid films of the presently disclosed technology may have a filmdensity 1.5-6 g/cm³, 1.5 to 2 g/cm³, 2 to 3 g/cm³, 3 to 4 g/cm³, 4 to 5g/cm³, or 5 to 6 g/cm³.

The presently disclosed technology provides a method of delivering atleast partially capped nanocrystals into the lubricated contact betweentwo surfaces formed by sliding and/or rolling said surfaces so as toproduce a pressure and/or shear stress on the lubricated surface andthereby forming a solid film, wherein the solid film is adhered to atleast one of the surfaces, the lubricant comprising at least partiallycapped, metal oxide nanocrystals having a mean size of 3 nm to 20 nm, 3nm to 5 nm, 5 nm to 10 nm, 10 nm to 15 nm, or 15 nm to 20 nm.

Methods of the presently disclosed technology provide solid films on atleast two surfaces that may be portions of a piston ring-cylinder linercontact, a cam and lifter contact, a contact between a rolling elementand races, gear teeth, or a hydrodynamic bearing shell and a rotor, or ahydrostatic bearing and stator or any other tribological contact surfacewith locally high pressures as described herein. The presently disclosedtechnology further provides a piston ring-cylinder liner contact, a camand lifter contact, a contact between a rolling element and races, gearteeth, or a hydrodynamic bearing shell and a rotor, or a hydrostaticbearing and stator, or any other tribological contact surface withlocally high pressures as described herein, containing a solid film ofthe presently disclosed technology.

BRIEF DESCRIPTION OF TABLES

TABLE 1: Surface parameters of the samples used in Example 1.

TABLE 2: Exemplary modulus and hardness measurement results of thetribofilm

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A: is an exemplary illustration of the reciprocating ball-on-flattester used in Example 1—schematic of contactconfiguration—reciprocating ball-on-flat.

FIG. 1B: is an exemplary illustration of the reciprocating ball-on-flattester used in Example 1—schematic of contactconfiguration-reciprocating ring-on-liner.

FIG. 2A: Profilometric images of optical profilometric image aslide-honed cylinder liner surface.

FIG. 2B: Profilometric images of optical profilometric image a topcompression ring surface.

FIG. 3A: A photo of the Micro-Pitting Rig (MPR) used in the examples.

FIG. 3B: A close-up photo of the MPR used in Example 1 shows thelubricant at rest covering the lower portion of the test rings.

FIG. 4: Provides a schematic of the MPR contact configuration.

FIG. 5A. Optical Images of Tribofilms formed by ball-on-flat test after1 minute.

FIG. 5B. Optical Images of Tribofilms formed by ball-on-flat test after5 minutes.

FIG. 5C. Optical Images of Tribofilms formed by ball-on-flat test after20 minutes.

FIG. 6A. Optical image of ball test scar after room temperatureball-on-flat test using PAO4+1 wt % capped ZrO2 nanoparticles (PAO ispoly-alpha-olefins).

FIG. 6B. Optical image of flat test track after room temperatureball-on-flat test using PAO4+1 wt % capped ZrO2 nanoparticles.

FIG. 7A: SEM-EDX (Scanning Electron Microscopy—Electron DispersionSpectroscopy) spectrum taken outside the flat wear track on the flatformed by 2 wt % capped ZrO2 nanoparticles in PAO oil showing Fe as thedominant element.

FIG. 7B: SEM-EDX spectrum taken inside the flat wear track showing Zr asthe dominant element.

FIG. 8A: Optical profilometer image and line scan (solid lines) of atribofilm formed by 1 wt % capped ZrO2 nanoparticles in PAO at 70° C. ona 52100 flat.

FIG. 8B: Optical profilometer line scan showing approximately 350 nmbuildup of tribofilm on the surface of the flat.

FIG. 8C: Region evaluated for the buildup rate of the tribofilm (box).

FIG. 9: An exemplary micrograph showing tribofilm formation on a linerafter a test at 100° C. using PAO10+1 wt % capped ZrO2 nanocrystals.

FIG. 10. EDX spectrum performed inside wear track of a flat tested withMobil 1 10W30 and 1 wt % capped ZrO2 nanoparticles.

FIG. 11: Evolution of tribofilm formation on the ring under for puresliding during an MPR test.

FIG. 12: Evolution of tribofilm formation on the ring up to 2 hoursduring an MPR test.

FIG. 13A: SEM image of an area inside the test track on the ring afteran MPR test.

FIG. 13B: EDX spectrum of an area inside the test track on the ringafter an MPR test.

FIG. 14A: SEM image of an area inside the test track on the ring focusedon a groove.

FIG. 14B: EDX spectrum of an area inside the test track on the ringfocused on a groove.

FIG. 15: A schematic of the AFM configuration used for generatingtribofilms.

FIG. 16A: The tribofilm growth volume as function of mean contactstress, in an AFM set up.

FIG. 16B: The tribofilm growth volume as function normal load, in an AFMset up, demonstrating stress-driven behavior.

FIG. 17A: An exemplary aerial view of the tribofilm generated by an AFM.

FIG. 17B: An exemplary top view of the tribofilm generated by an AFM.

FIG. 17C: An exemplary line scan of the tribofilm generated by an AFM.

FIG. 18: Cross-sectional imaging of the zirconia tribofilms at differentmagnification showing polycrystalline structure.

FIG. 19A: A cross-sectional TEM image of a tribofilm formed by AFM.

FIG. 19B: Cross-sectional EDX mapping of the same tribofilm showing thatzirconia tribofilms are deficient in carbon-containing capping agentsand the composition of Fe and Zr formed compositional gradients insidethe tribofilm at different depths.

FIG. 20A: Growth rates and cycles to tribofilms nucleation plotted forvarious sub-ambient test temperatures. Under tested contact conditions,tribofilm growth is observed for all temperatures between −25° C. and25° C. although some variation in growth rate is observed.

FIG. 20B: Growth rates and cycles to tribofilms nucleation plotted forvarious sub-ambient test temperatures—reducing interfacial temperaturereduces the cycles-to-nucleation resulting in a more rapid growthinitiation.

FIG. 21: Cross-sectional TEM image of a tribofilm formed by AFM using aPAO4 base oil consisting of 9 wt. % zirconia with 0.8% wt. % ZDDP.Cross-sectional images show that ZDDP restricts grain coalescence andgrowth normally seen in pure zirconia tribofilms.

FIG. 22: Cross-sectional TEM image of a tribofilm formed by AFM using aPAO4 base oil consisting of 9 wt. % zirconia with 0.8% wt. % ZDDP (left)and EDX analysis performed across the cross-section of this tribofilms(right). EDX confirms the presence of zirconia in the tribofilms, aswell as phosphorous, sulfur and zinc, which confirms that thesetribofilms consist of a ZDDP phase mixed with zirconia.

The present disclosure provides the following additional embodiments.

EXAMPLES

Test Equipment

Reciprocating Rig

Experiments were performed with two contact configurations (ball-on-flatand ring-on-liner) on the same reciprocating tribometer. Theball-on-flat configuration used 52100 steel counterfaces and 12.7-mm(½-in.) diameter balls (Grade 25) sliding against mirror-polished flats(Sq=10 nm). The load of 15.6 N produced an initial peak Hertzian contactpressure of 1 GPa. The ring-on-liner configuration used specimensextracted from components in a commercial heavy-duty diesel engine.During all machining operations to extract test specimens, the originalsurfaces of the piston rings and cylinder liners were protected in orderto retain the original surface roughness and honing pattern. The linerswere gray cast iron with a typical honing pattern, and the ring wassteel that had been coated with CrN by physical vapor deposition (PVD).The cylinder liner was mounted onto a reciprocating table on the bottomof the test rig, while the piston ring was stationary. The curvature ofthe ring was adjusted so that a Hertzian contact width of 10 mm wasachieved. A load of 200 N produced a contact pressure of approximately110 MPa, which is similar to the contact pressure experienced by the topcompression ring at the top dead center (TDC) position in severeservice. Schematics for the two contact configurations are shown in FIG.1A and FIG. 1B. FIG. 2A and FIG. 2B shows profilometric images of thecylinder liner and top-compression ring surfaces, respectively. Theirsurface parameters are given in Table 1.

The cylinder liner was mounted onto a reciprocating table on the bottomof the test rig, while the piston ring was stationary. The curvature ofthe ring was adjusted so that a Hertzian contact width of 10 mm wasachieved. A load of 200 N produced a contact pressure of approximately110 MPa, which is similar to the contact pressure experienced by the topcompression ring at TDC in severe service.

A small amount of oil (0.3 ml) was applied at the interface of the testcomponents to create a thin layer at the start of each test. The testswere conducted at 1 Hz reciprocating frequency for 1 hour using a strokelength of 20 mm. Heating elements were embedded into the reciprocatingtable, and the temperature was controlled by a temperature control unit.Tests were performed at 70° C., 100° C., 130° C., and 160° C.respectively.

Micro-Pitting Rig (MPR)

FIG. 3A is a photo of the Micro-Pitting Rig (MPR) available at ANL. Itconsists of a center roller in contact with three larger rings. FIG. 3Bshows the lubricant at rest covering the lower portion of the testrings. The lubricant is supplied to the contact via splash lubrication.Both the rings and the barrel are uni-directionally satin ground. Thecontacting area is flat and approximately 1 mm wide. The roughness of aring is approximately 150 nm. The rotation speed of the rings and rollerare independently controlled allowing for a range of slide-to-roll (SRR)speed ratios. The load, speed, temperature, and SRR can all becontrolled and set to a condition that is relevant for replicating geartooth contact. Additionally, the materials and surface roughness of thesamples can be tailored to match that of the gear components. During atest, the MPR is capable of measuring the friction force between theroller and the rings, as well as the vibration developed at the contact,indicating the severity of the accumulated surface damage. After a test,the roller and ring samples are analyzed to quantify the amount ofsurface wear. Further examination of the samples can be used tocharacterize the protective tribofilm that formed on the surface fromthe lubricant additives. MPR tests were performed to evaluate thefriction and wear (and/or pitting) performance of lubricants formulatedwith ZrO2 nanocrystal additives.

Characterization Techniques

Surface Profilometry

An interferometric non-contact optical profilometer (Bruker®, ContourGT,San Jose, Calif.) was used for measuring roughness, finish, and textureof a surface. Due to optical interference, micrographs of thintransparent films show colors that are a function of film thickness. Inorder to show the true surface of a tribofilm, the test components werecoated with a thin layer of gold.

Microscopy

The wear tracks on the flats and cylinder liners after the tests wereexamined with an Olympus STM6 optical microscope, an FEI Quanta 400Fscanning electron microscope (SEM), a Hitachi S-4700-II SEM, bothequipped with energy dispersive x-ray spectroscopy (EDX) capability.

Nano-Indentation

A nanoindenter (Hysitron TI-950 Tribo-Indenter) was used to determinethe hardness and modulus of these tribofilms formed on surfaces, underdisplacement control using a standard Berkovich tip. The same tip wasused under scanning probe microscopy (SPM) mode to image the surfacetopography. The nanoindenter monitors and records the load anddisplacement of the indenter during indentation with a force resolutionof about 1 nN and a displacement resolution of about 0.2 nm. The sampleswere placed on a magnetic horizontal holder and positioned with the aidof an optical microscope located above the sample. The area functionparameters of the tip were calibrated using a fused quartz sample, andtip-shape calibration is based on determining the area function of theindenter tip.

Example 1

Capped nanocrystals can be dispersed into base oil with multiple cappingagents at least as high as 10 wt % without significantly affecting theviscosity and appearance of the oil. Concentrations of 0.5 wt. %, 1 wt.%, 2 wt. % and 10%, three different, capping agents, temperature (25°C., 70° C., 130° C., 160° C.), time (5 mins, 20 mins, 60 mins, 4 hrs, 24hrs), and type of oil were parameters that were investigated.

An important observation is the formation of a unique tribofilm by ZrO2nanocrystal additives regardless of temperature. A tribofilm started toform on the flat during the ball-on-flat test only 1 minute after thetest started, and a thick and dense (as judged by optical profilometry)tribofilm was fully formed on the flat 20 minutes into the test, asshown in FIG. 5. Due to the relatively long stroke length, the flatexperienced much less rubbing than the ball, on which a thick and densetribofilm was fully formed after 20 minutes.

The formation of a tribofilm was also observed at room temperature usingPAO4 as base oil with 1 wt % capped ZrO₂.nanocrystals The ball test scarand flat test track are shown in FIG. 6A and FIG. 6B, respectively.

A prominent zirconium peak in the SEM-EDX spectrum was found in the weartrack (FIG. 7B) but it was absent outside the wear track (FIG. 7A),indicating that the tribofilm was zirconium-rich and the tribofilm hadindeed originated from the ZrO₂ nanocrystal additives.

The tribofilms were semi-transparent so a thin gold layer was coated onthe ball and flat by thermal evaporation to assure the accuracy whenexamined with optical profilometer. An optical image of a tribofilmobtained by the optical profilometer is shown in FIG. 8A. Instead of anet loss of material characteristic of wear, there was actually a netincrease of material on the wear track. Line scans (vertical solid line)across the film revealed that the tribofilm has a height of about 350 nmabove the flat surface (FIG. 8C).

Quantitative evaluation of the area marked in the FIG. 8B by a solidrectangle showed a net nanocrystal-based tribofilm build-up rate of62,700 μm3 per mm of sliding distance per hour, approximately 1/300 ofthe total nanocrystal loading included in the amount of oil used in thetests. This indicated that there are significant amounts of nanocrystalsleft to continue re-generating the tribofilm. The tribofilm was alsorelatively smooth, the root mean square (RMS) roughness of the tribofilmwas measured to be 170 nm while for the mirror polished flat the valuewas 40 nm.

A tribofilm was also formed on liner segments in ring-on-liner tests ata range of conditions as shown in an exemplary image in FIG. 9.

The modulus and hardness of the tribofilm were also measured usingnano-indentation, and exemplary results are shown in Table 2, togetherwith the results of the steel flat. The tribofilm possess veryimpressive modulus and hardness, only ˜30% less than 52100 in bothcases. A tribofilm that is hard, but slightly softer than the surfacematerial can provide sufficient load bearing capability as a rubbingsurface while serving as a protective, regenerative layer if the stressis too high.

A tribofilm also formed by adding capped ZrO₂ nanoparticles in a fullyformulated oil (Mobil 1 10W30). The presence of Zr was confirmed withEDX after a test. The result is shown in FIG. 10.

A tribofilm formed under pure rolling conditions in an MPR test, at aload of 200 N, speed of 2 m/s, and a temperature of 70° C., as early as15 minutes (143,000 cycles), continued to grow over time, and becamemore uniform throughout the test. The film was maintained up to 24 hoursof testing (13.8 million cycles). The evolution of the tribofilm isshown in FIG. 11.

A tribofilm also formed under a combination of rolling and slidingconditions in an MPR using capped ZrO2 nanocrystals loaded mineral oil.The evolution of the tribofilm is shown in FIG. 12.

FIG. 13A showed an SEM image of part of the tribofilm inside the testtrack on the ring after the MPR test. And EDX analysis was performed andindicated the presence of Zr on the test track on the ring, as shown inFIG. 13B. Also, grooves were observed on the tribofilm and an SEM imageof the groove is shown in FIG. 14A, and EDX inside the grooves showed noZr (FIG. 14B) which means that the grooves are not filled with ZrO2nanocrystals.

Example 2

Tribofilms with the capped ZrO2 nanocrystals were also generated in anatomic force microscope (AFM) at the interface formed by a steelmicrosphere (ranging between 10 and 100 μm in diameter) against either a52100 steel substrate, or a silicon substrate or a yttria-stabilizedzirconia substrate (illustrated in FIG. 15). The contact stress at thesliding contact was varied between 0.1 GPa and 1 GPa. Zirconiatribofilms exhibit a stress-driven growth process where increasing thecontact stress increases the thickness of the tribofilms (FIG. 16).Increasing surface roughness increases the rate of tribofilm growth.These tribofilms are strongly bound to the substrate and resist removalduring continued sliding with the AFM probe in either base oil or in drysliding.

Using the AFM, tribofilms with lateral dimensions as small as 2 μm andas large as 50 μm were generated, with local thickness varying from 10nm to 200 nm (example shown in FIG. 17).

Tribofilms in the AFM were generated in concentrations of cappedzirconia nanoparticles ranging from 0.01 wt. % in PAO4 to 10 wt. % inPAO4. Additionally, tribofilms were generated in other base stocks,including mPAO SYN65.

Using the AFM, tribofilms were generated at temperatures ranging from−25° C. to 130° C. (FIG. 20 shows a range of temperature from −25° C. to25° C.).

Tribofilm microstructure and chemical composition were analyzed byperforming focused-ion beam (FIB) milling to produce a cross-sectionalsample of the tribofilm, followed by observation in scanning electronand transmission electron microscopes (SEM/TEM).

Cross-sectional imaging of the tribofilms show a nearly fully densemicrostructure with no observable voids. Diffraction analysis confirmsthat the tribofilms consist of a mostly polycrystalline structure,identified to be zirconia. Through cross sectional imaging, evidence ofgrain growth and coalescence of individual 5 nm zirconia nanoparticlesis also seen, as shown in FIG. 18.

Through these cross-sectional images and accompanying chemicalspectroscopy (such as EDX, EELS and FTIR), it is confirmed that zirconiatribofilms are deficient in carbon, indicating that tribologicalstresses during sliding result in the removal of capping agents prior totribofilms formation (FIG. 19).

The mechanism of tribofilms growth as deduced from these images is asfollows: nanoparticles undergo selective removal of surface ligands,i.e. capping agents, at the sliding contact due to tribologicalstresses. In the absence of dispersing ligands, the nanoparticlesinteract strongly with the substrate and each other and tribologicalstresses cause the nanoparticles to bind strongly to the substrate andto each other, resulting in the nucleation and growth of a compacttribofilm. As the film grows, stress-driven grain coarsening occurs.Tribofilms generated in the sliding contact of the AFM show superiormechanical properties. The modulus and hardness of these films wasmeasured to be about 160 GPa and 7.3 GPa, respectively. These valuesapproach known literature values of bulk zirconia.

Tribofilms in the AFM were also generated with a mixture of cappedzirconia nanoparticles mixed with zinc dialkyldithiophosphates (ZDDP)anti-wear additives. In these measurements, zirconia was added to a PAO4base oil in either 9 wt. %, 1 wt. %, 0.1 wt. % or 0.01 wt. %, and mixedwith 0.8 wt. % ZDDP. With this oil containing both ZDDP and cappedzirconia nanoparticles, measurements were made at a variety oftemperatures including 25° C., 15° C., 5° C. and −5° C. Other parametersfor these AFM tests (load, speed, etc.) were similar to those indicatedin example 2. For all tested temperatures, and for all concentrations ofcapped zirconia mixed with ZDDP additive, a tribofilm growth andformation was observed in the AFM. Similar results are expected forlower temperatures, such as −15° C. and −25° C. These zirconia-ZDDPtribofilms were morphologically similar to pure zirconia tribofilms.However, for identical test conditions and durations, the zirconia-ZDDPtribofilms generated had a significantly higher thickness (i.e. volume)compared to pure zirconia tribofilms. In addition, tribofilms formedwithin the AFM with zirconia-ZDDP mixed in PAO4 were found to nucleateon the surface much more rapidly in comparison to pure zirconiatribofilms, which resulted in a significantly rapid tribofilms growthinitiation.

Cross-sectional imaging of tribofilms formed in oils containing bothzirconia and ZDDP exhibit zirconia nanocrystal sizes of 5 nm, whichindicate that ZDDP is effective in inhibiting grain growth andcoalescence as is seen in pure zirconia tribofilms (FIG. 21). Chemicalspectroscopy of FIB/SEM cross-sections of these ZDDP-zirconia tribofilmsindicate the presence of both zirconia, as well as zinc, phosphorous andsulfur, and a relative high concentration of carbon, which confirm thatthese tribofilms consist of a distinct zirconia phase as well as adistinct ZDDP phase (FIG. 22).

TABLE 1 Surface parameters of the samples used in Example 1. Liner Liner(slide-honed) (slide-honed) PVD CrN Ring PVD CrN Ring 10 × 0.55 50 × 1.010 × 0.55 50 × 1.0 Sa (μm) 0.662 0.175 0.822 0.208 Sq (μm) 0.936 0.2241.451 0.268 Ssk (—) −2.132 −0.655 −0.991 0.77 Sku (—) 10.238 4.867 41.5120.88 Sp (μm) 2.951 1.363 36.524 10.297 Sv (μm) −10.932 −1.869 −33.041−3.407 Sz (μm) 13.883 3.231 69.565 13.703

TABLE 2 Exemplary Modulus and Hardness Measurement Results of theTribofilm Surface Modulus (GPa) Hardness(GPa) Tribofilm 148.40 7.0452100 steel 216.83 11.48The contents of all references referred to herein are incorporated intheir entirety in this disclosure.

We claim:
 1. A lubricant comprising zirconia nanoparticles, wherein thezirconia nanoparticles are capped with at least one capping agent, andwherein the lubricant has a minimum transmittance of larger than 50%when measured in a cuvette with a 10 mm path length when the dispersioncontains 10% by weight nanoparticles in the lubricant.
 2. The lubricantof claim 1, wherein the lubricant comprises an oil, a grease or asynthetic, mineral or natural lubricant and/or contains at least one ofsynthetic hydrocarbon, an ester, a silicone or a polyglycol.
 3. Thelubricant of claim 1, wherein the lubricant is a stable dispersion andexhibits less than 10% change in optical transmittance, when measured ina cuvette with 10 mm optical path, after 1 month of storage to greaterthan 3 years storage.
 4. The lubricant of claim 1, wherein the lubricantfurther comprises at least one lubricant additive selected from thegroup consisting of anti-wear (AW) additives, friction modifiers (FM),anti-oxidants, extreme pressure (EP) additives, anti-foaming agents,detergents, dispersants, and pour point depressants.
 5. The lubricant ofclaim 1, wherein the lubricant further comprises zincdialkyldithiophosphates (ZDDP).
 6. The lubricant of claim 1, wherein thelubricant has a viscosity in the range of 2 to 1000 mPas (cP) at atemperature of 100° C.
 7. The lubricant of claim 1, wherein thelubricant is a crankcase lubricant for internal combustion engines, alubricating oil for geared transmissions in vehicles and wind turbinedrivetrains, or a lubricant for rolling bearing elements.
 8. Thelubricant of claim 1, wherein the zirconia nanoparticles have a sizewhich is 4-20 nm.
 9. The lubricant of claim 1, wherein the zirconiananoparticles are present in an amount of 0.01 to 10 wt. %, based ontotal weight of the lubricant.
 10. The lubricant of claim 1, wherein thenanoparticles comprise nanocrystals.
 11. The lubricant of claim 5,wherein the zinc dialkyldithiophosphates (ZDDP) is present in an amountof 0.01 to 2 wt. %, based on total weight of the lubricant.
 12. Thelubricant of claim 1, wherein the nanoparticles in use undergo selectiveremoval of the at least one capping agent due to tribological stresses.13. A tribofilm which comprises the lubricant of claim 1, wherein the atleast one capping agent is removed due to tribological stresses.